Indonesia has deployed nearly 21,000 personnel to fight forest fires raging in its northern islands, as thick smoke cloaks much of the region. Hazardous smog has blanketed parts of northern Indonesia, Singapore and Malaysia.

South-east Asia has suffered for years from annual bouts of smog caused by slash-and-burn practices on Sumatra and Kalimantan islands, and the fires have been exacerbated this year by the effects of the El Nino weather phenomenon, as a prolonged dry season has parched the soil, fuelling the flames.

These interactive before-and-after photos show how visibility in neighbouring Singapore has been dramatically affected.

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People relax in an infinity pool overlooking the skyline of the central business district of Singapore on 14 September and 10 July 2015 (Edgar Su/Reuters)

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Vehicles travel on the causeway from Singapore to to Malaysia on 29 September 2015 and 5 May 2015 (Edgar Su/Reuters)

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People stand in a capsule on the Singapore Flyer observatory wheel on 10 September 2015 and 16 July 2015 (Edgar Su/Reuters)

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The Singapore F1 Grand Prix night race Marina Bay street circuit is seen on 14 September 2015 and 17 September 2013 (Edgar Su/Reuters)

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The Flower Dome and Cloud Forest conservatories of Gardens by the Bay in Singapore on a hazy day on 10 September 2015 and on a clear day on 16 July 2015 (Edgar Su/Reuters)

The smog is caused by firms and smallholder farmers clearing land for palm or pulp and paper. Major plantation companies like Asia Pulp and Paper say they have a "zero burning" policy but have often been criticised by green groups for not doing enough to stop the haze.

A state of emergency has been declared in Indonesia's Riau and Central Kalimantan provinces. More than 135,000 Indonesians are reported to be suffering from respiratory diseases. Schools in Riau province are shut. Students are expected to return to school on 30 September, but if the situation worsens, schools could remain closed.

An officer from the local Disaster Management Agency attempts to extinguish a fire in Sei Rambutan, Ogan Ilir, South SumatraNova Wahyudi/Antara Foto/ReutersIndonesian police and firefighters extinguish burning peat land in the district of Kapuas in the Central Kalimantan province on BorneoRomeo Gacad/AFPTroops extinguish a fire in a peat land forest in the Jabiren Raya district of Central Kalimantan province on BorneoRomeo Gacad/AFPIndonesian soldiers and firefighters are engulfed in a dust cloud kicked up by a helicopter taking part in a firefighting operation in Banjarbaru in Southern Kalimantan province on BorneoRomeo Gacad/AFPA couple have their wedding photos taken in the backdrop of the haze-shrouded central business district in SingaporeEdgar Su/ReutersBoats carrying goods and passengers are seen on the Batang Hari River covered in a thick haze in Jambi on the Indonesian island of SumatraWahdi Septiawan/Antara Foto/ReutersA mother adjusts her daughter's glasses as they head home from school in Kertapati district in Palembang, on the Indonesian island of SumatraBeawiharta/ReutersMalaysia's landmark Petronas Twin Towers are covered in haze in Kuala LumpurMohd Rasfan/AFPThe sun is dimly seen through a blanket of haze at the Garden by the Bay in SingaporeRoslan Rahman/AFPPeople ride their motorcycles along a hazy street street near Bagan Siapi-Api port, on the Indonesian island of SumatraBeawiharta/ReutersCable cars travelling towards the island resort of Sentosa are shrouded by haze in SingaporeEdgar Su/ReutersA helicopter belonging to the National Agency for Disaster Management waits for improved visibility at Palembang airport on the Indonesian island of SumatraNova Wahyudi/Antara Foto/Reuters

Indonesia has faced criticism for turning down offers of help from Singapore. Indonesian officials, including vice president Jusuf Kalla, have repeatedly said they have enough resources to handle the crisis. Kalla said that neighbouring countries "enjoy 11 months of clean fresh air from Indonesia" and suggested that it is not a big deal if they suffer from the haze for one month.